Temporal patterns of sexual behavior in rats may have significant adaptive value. If so, it should have a heritable component, and be similar in both male and female animals. A measure of temporal pattern applicable to both sexes has been developed. It is defined as the length of time between two sequential entries of the test animal into the space of an observation chamber occupied by a tethered stimulus animal. A breeding protocol involving sib selection has been developed for producing animals with high and low measures and determining (a) the fecundity of matings between these animals and (b) the heritabilities of the measures. To determine the correlation of this measure and other measures of sexual behavior, a series of typically male and female behaviors will also be employed.